Method And Apparatus For Locating Mobile Receivers Using A Wide Area Reference Network For Propagating Ephemeris

ABSTRACT

A method and apparatus for distribution and delivery of global positioning system (GPS) satellite telemetry data using a communication link between a central site and a mobile GPS receiver. The central site is coupled to a network of reference satellite receivers that send telemetry data from all satellites to the central site. The mobile GPS receiver uses the delivered telemetry data to aid its acquisition of the GPS satellite signal. The availability of the satellite telemetry data enhances the mobile receiver&#39;s signal reception sensitivity.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to signal processing in GPS receivers. Inparticular, the present invention relates to a method and apparatus fordelivering satellite data to GPS receivers to enable a GPS receiver toacquire and lock on to GPS satellite signals in low signal strengthenvironments (e.g., indoors).

2. Description of the Background Art

Conventional GPS receivers require an inordinate amount of time toacquire and lock onto the satellite signals. Then, once locked, a GPSreceiver extracts telemetry data (almanac and ephemeris) from thesignal. From these data the GPS receiver can calculate information thatenhances its ability to lock onto the satellite signal. A relativelyhigh signal strength satellite signal is necessary to enable the systemto achieve an initial lock. Once the GPS signal is acquired, the signalstrength must remain high while the almanac and/or ephemeris data isextracted from the satellite signal. Any severe attenuation of thesignal can cause a loss of lock and the signal will requirere-acquisition. As such, the system has an inherent circularity thatmakes it difficult or impossible for GPS receivers to acquire signals inlow signal strength environments.

To aid initial acquisition of the satellite signal, many GPS receiversstore a copy of the almanac data, from which the expected Dopplerfrequency of the satellite signal can be calculated. Several techniqueshave been developed to calculate useful information at a separate GPSreceiver and then transmit this data to another GPS receiver. U.S. Pat.No. 6,064,336, issued May 16, 2000, collects almanac data at a separateGPS receiver, then transmits the almanac data to a mobile receiver. Themobile receiver then uses the almanac data to compute the expectedDoppler frequency of the satellite signal, thus aiding in initial signalacquisition.

The advantage of receiving the almanac is that each GPS satelliterepeatedly transmits a complete almanac containing orbit information forthe complete GPS constellation, thus a single GPS receiver, tracking anysatellite, can collect and propagate the almanac for all satellites inthe constellation. The disadvantage of using the almanac is that it is afairly rough model of the satellite orbit and satellite clock errors,thus the almanac is only useful in reducing the frequency uncertaintyand cannot be used to enhance receiver sensitivity by reducing thesearch window of code-delay uncertainties.

If a GPS receiver had a complete set of ephemeris data for allsatellites in view, before said receiver attempted to lock onto thosesatellites, the receiver would have significantly improved acquisitiontimes and enhanced sensitivity. This is because the ephemeris datacontains an accurate description of the satellite position, velocity,and clock errors; and the GPS receiver can use this data to increase itssensitivity by reducing significantly the search windows for frequencyuncertainty and code-delay uncertainty. The disadvantage of theephemeris is that each satellite only transmits its own ephemeris; thusa single GPS receiver cannot collect and propagate ephemeris for all thesatellites in the constellation.

Therefore there is a need in the art for a GPS receiver system thatpropagates satellite ephemeris for all satellites in the constellation,thereby enhancing the speed of acquisition and signal sensitivity ofmobile receivers.

SUMMARY OF THE INVENTION

The invention comprises a method and apparatus for distribution anddelivery of the Global Positioning System (GPS) satellite ephemerisusing a communication link between a central site and a wide areanetwork of GPS receivers. The wide area network of GPS receiverscollects the ephemeris data that is transmitted by the satellites andcommunicates the data to the central site. The central site delivers theephemeris to the mobile receiver. The mobile GPS receiver uses thedelivered data to enhance its sensitivity in two ways. First, the dataallows the receiver to detect very weak signals that the receiver wouldnot ordinarily be able to detect, and second, the GPS receiver does nothave to track the satellite signals for very long before a position canbe calculated.

In one embodiment of the invention, the satellite ephemeris data isretransmitted without manipulating the data in any way. The GPS receivermay then use this data exactly as if the receiver had received the datafrom the satellite. In another embodiment, a satellite pseudo-rangemodel is computed at the central site from the ephemeris data, and thispseudo-range model is transmitted to the GPS receiver. The pseudo-rangemodel has the characteristic that the model is more concise than thecomplete ephemeris. As such, the GPS receiver does not have to performas many calculations when using the pseudo-range model as when using thecomplete ephemeris.

BRIEF DESCRIPTION OF DRAWINGS

The teachings of the present invention may be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts an architecture for a wide area reference station networkin accordance with the present invention;

FIG. 2 depicts a GPS orbital sphere;

FIG. 3 depicts the intersection of the GPS orbital sphere and thehorizon planes of three reference stations;

FIGS. 4A and 4B depict the intersection of the GPS orbital sphere andthe horizon planes of four reference stations;

FIG. 5 depicts a flow diagram of a method of generating pseudo-rangemodels;

FIG. 6 is a graph illustrating the timing (pseudo-range) and frequency(pseudo-range rate) uncertainty for a mobile GPS receiver, and theimprovement in sensitivity that is gained by reducing both theseuncertainties;

FIG. 7 depicts a flow diagram of a method of searching through the time(pseudo-range) and frequency (pseudo-range rate) windows; and

FIG. 8 depicts a flow diagram of a method for using pseudo-rangeinformation of satellites having high signal strength to improvereceiver sensitivity for signals received from satellites having lowsignal strength.

DETAILED DESCRIPTION OF THE INVENTION

To facilitate understanding, the description has been organized asfollows:

Overview, introduces each of the components of the invention, anddescribes their relationship to one another.

Global Tracking Network, describes how a worldwide network of trackingstations is constructed and deployed to ensure that all satellites aretracked at all times.

Ephemeris Processing, describes an embodiment of the invention thatprovides a more compact, and simpler, model of the satellite ephemeris.

Signal Detection, describes how the retransmitted satellite ephemerisdata is used in a GPS receiver to detect signals that would otherwise beundetectable.

Sensitivity Enhancement, describes how the two strongest satellitesignals may be used to compute the time and correlator offset at themobile receiver. This information is, in turn, used to enhancesensitivity for weaker GPS signals that are received by the mobilereceiver.

Overview

FIG. 1 depicts one embodiment of a global positioning system (GPS)satellite data distribution system 100 comprising:

A reference station network 102 comprising a plurality of trackingstations 104 ₁, 104 ₂, . . . 104 _(n) coupled to one another through acommunications network 105. The reference stations 104 are deployed overa wide area and contain GPS receivers 126 so that ephemeris may becollected from all satellites 106 within a global network of satellitese.g., the global positioning system (GPS). Ephemeris informationcomprises a 900 bit packet containing satellite position and clockinformation.

A central processing site 108 that collects the ephemeris from thetracking stations 104 comprises an ephemeris processor 128 that removesduplicate occurrences of the same ephemeris, and provides the latestephemeris data for redistribution to mobile GPS receivers 114 and 118.

A communications link 120 from the central processing site to the mobileGPS receiver 114. The link 120 may be a landline 110, or other directcommunications path that couples the mobile GPS receiver 114 directly tothe central processing site 108. Alternatively, this link may haveseveral parts, for example: a landline 112 to a wireless transmitter116, and a wireless link 122 from the transmitter 116 to a mobilereceiver 118.

A mobile GPS receiver 114 or 118 that uses the redistributed ephemerisdata (or a modified form thereof) to aid the receiver in detecting GPSsignals from satellites 106 in a satellite constellation.

A position processor 130, where the position of a GPS receiver 114 or118 is calculated. This could be the GPS receiver itself, the centralprocessing site 108, or some other site to which the mobile GPSreceivers send the measurement data that has been obtained from thesatellites 106.

In operation, each of the satellites 106 continually broadcast ephemerisinformation associated with a particular satellite. To comprehensivelyand simultaneously capture the ephemeris data of all the satellites 106in the constellation, the network 106 is spread worldwide.

To obtain all the ephemeris data, three or more tracking stations 104are needed. Each of the 28 satellites has an orbit inclined at 55degrees relative to the equator of the earth. As such, no satellite evertravels outside of a plus or minus 55 degree range on an orbital sphere.Consequently, three stations placed 120 degrees apart and lying exactlyon the equator of the earth, would have all the satellites in view.However, placing reference stations at or close to those exact locationson the equator is impractical. To place reference stations in largecities around the world, a realistic, minimum number that will achieveviewing of all the satellites 106 is four.

Each of the tracking stations 104 contains a GPS receiver 126 thatacquires and tracks satellite signals from all satellites 106 that arein view. The stations 104 extract the ephemeris information thatuniquely identifies the position of each satellite as well as satelliteclock information e.g., a 900 bit packet with a GPS signal. Theephemeris information is coupled to the central processing site 108 via,for example, a terrestrial land line network 105.

The central processing site 108 sends all or part of the ephemerisinformation to one or more mobile GPS receivers 114 and 118. If thecentral processing site knows the approximate position of the mobile GPSreceiver, the central processing site 108 may only send the ephemerisinformation for satellites that are presently (or about to be) in viewof the mobile GPS receiver 114 or 118. The ephemeris information can becoupled directly through a land line 110 or other communication path(e.g., internet, telephone, fiber optic cable, and the like).Alternatively, the ephemeris information can be coupled to a mobile GPSreceiver 118 through a wireless system 116 such as a cell phone,wireless Internet, radio, television, and the like. The processing andutilization of the ephemeris information is described below (seeEPHEMERIS PROCESSING and SIGNAL DETECTION).

Global Tracking Network

The global GPS reference network 102 has stations 104 arranged such thatall satellites are in view all the time by the tracking stations 104 inthe network 102. As such, the ephemeris for each satellite 106 isavailable to the network in real time, so that the network, in turn, canmake the ephemeris, or derived pseudo-range models, available to anymobile receiver that needs them.

The minimum complete network of reference stations comprises threestations, approximately equally placed around the earth, on or close tothe equator. FIG. 2 shows the GPS orbital sphere 202 surrounding theearth 204, and an indication 206 of all orbits of the satellites. FIG. 3shows the intersection of the horizon planes of 3 tracking stations,(denoted A, B, and C), with the GPS orbital sphere. In FIG. 3, theorbital sphere is shaded gray in any region above the horizon of atracking station. Regions on the orbital sphere above the horizons oftwo tracking stations are shaded slightly darker. The orbital sphere iswhite in the regions, above and below 55 degrees, where there are no GPSsatellites. From FIG. 3, it is clear that any point on any GPS orbit isalways above the horizon of at least one reference station A, B or C.

It is not commercially or technically practical to place referencestations around the equator. Preferred sites are major cities with goodcommunications infrastructure to enable the ephemeris to be coupled tothe control processing site via a reliable network. When the referencestations are moved away from the equator, more than three stations areneeded to provide coverage of all satellites all the time. However, itis possible to create a network of only four reference stations withcomplete coverage of all GPS satellites all the time, and with the fourstations located in or near major cities. For example, the stations maybe placed in Honolulu, Hi. (USA), Buenos Aires (Argentina), Tel Aviv(Israel) and Perth (Australia). FIGS. 4A and 4B show the intersection ofthe horizon planes of these stations with the GPS orbital sphere. Anypoint of any GPS orbit is always above the horizon of at least one ofthe reference stations. FIGS. 4A and 4B show the orbital sphere viewedfrom two points in space, one point (FIG. 4A) in space approximatelyabove Spain, and the other (FIG. 4 b) from the opposite side of thesphere, approximately above New Zealand. The figure is shaded in asimilar way to FIG. 3. Gray shading shows regions of the GPS orbitalsphere above the horizon of at least one tracking station and darkergray regions represent portions of the orbital sphere accessible to twostations.

Ephemeris Processing

The ephemeris is used to compute a model of the satellite pseudo-rangeand pseudo-range rate. From the pseudo-range rate the mobile GPSreceiver can calculate the Doppler frequency offset for the satellitesignal. The computation of the pseudo-range model can be done at themobile receiver, or at the central processing site. In the preferredembodiment the pseudo-range model is computed at the central site asfollows.

FIG. 5 depicts a flow diagram of a method 500 for generating apseudo-range model. At step 502, the ephemeris data from all thetracking stations is brought to the central processing site. Ephemerisdata is transmitted continually by all satellites, mostly this isrepeated data; new ephemeris is typically transmitted every 2 hours. Theephemeris is tagged with a “Time of Ephemeris”, denoted TOE. This tagindicates the time at which the ephemeris is valid. Ephemeriscalculations are highly accurate within 2 hours of TOE. A satellitefirst transmits an ephemeris 2 hours ahead of the TOE, thus anyephemeris is highly accurate for a maximum of four hours.

At step 506, the central processing site keeps all the ephemeris datawith TOE closest to the time T at which the mobile receiver requiresephemeris (or a pseudo-range model). Time T is provided by the mobilereceiver at step 504. Usually T will be the current real time, however,it could be a time up to 4 hours in the future for mobile receivers thatare collecting ephemeris/pseudo-range models in advance of when theyneed them. T could also be a time in the past, for mobile receiversprocessing previously stored data.

At step 508, the central processing site then calculates the satellitepositions at time T. In the preferred embodiment, this is performedusing the equations provided in the GPS Interface Control Document,ICD-GPS-200-B.

At step 512, the central processing site obtains the approximateposition of the mobile GPS Receiver. In the preferred embodiment, themobile GPS receiver communicates with the central processing site over awireless communications link, such as a 2-way paging network, or amobile telephone network, or similar 2-way radio networks. Such 2-wayradio networks have communication towers that receive signals over aregion of a few miles. The central processing site obtains the referenceID of the radio tower used to receive the most recent communication fromthe mobile GPS. The central processing site then obtains the position ofthis radio tower from a database. This position is used as theapproximate mobile GPS position.

In an alternative embodiment, the approximate position of the mobile GPSreceiver may be simply the center of the region served by a particularcommunications network used to implement this invention.

In another alternate embodiment, the approximate position of the mobileGPS receiver may be the last known point of said receiver, maintained ina database at the central processing site.

It is understood that many combinations and variants of the abovemethods may be used to approximate the mobile GPS receiver position.

Having calculated the satellite positions, and obtained the approximateuser position, the central processing site computes (at step 510) whichsatellites are, or will soon be, above the horizon at the mobile GPSreceiver. For applications requiring simply the redistribution of theephemeris data, at step 514, the central processing site now outputs theephemeris for those satellites above, or about to rise above, thehorizon.

In the preferred embodiment, a pseudo-range model is computed thatcomprises: time T, and, for each satellite above, or about to riseabove, the horizon: the satellite PRN number, pseudo-range, pseudo-rangerate, and pseudo-range acceleration.

To compute a pseudo-range model, the central processing site firstcomputes at step 516 the pseudo-ranges of all satellites above, or aboutto rise above, the mobile GPS receiver horizon. The pseudo-range is thegeometric range between the satellite and the approximate GPS position,plus the satellite clock offset described in the ephemeris.

At step 518, the pseudo-range rate may be computed from the satellitevelocity and clock drift. Satellite velocity may be obtained directly bydifferentiating the satellite position equations (in ICD-GPS-200-B) withrespect to time. In an alternative embodiment, satellite velocity may becomputed indirectly by computing satellite positions at two differenttimes, and then differencing the positions.

In another alternative embodiment, the pseudo-range rates may becomputed indirectly by computing the pseudo-ranges at two differenttimes, and then differencing these pseudo-ranges.

At step 520, the pseudo-range acceleration is then computed in a similarfashion (by differentiating satellite velocity and clock drift withrespect to time, or by differencing pseudo-range rates).

The complete pseudo-range model is then packed into a structure andoutput to the mobile GPS receiver at step 522.

The mobile GPS receiver may use the pseudo-range model for the period ofvalidity of the ephemeris from which it was derived. To apply thepseudo-range model at some time after time T, the mobile receiverpropagates the pseudo-ranges and range rates forward using the rate andacceleration information contained in the pseudo-range model.

In an alternative embodiment, the central processing site propagates theunaltered ephemeris 519 and the derivation of the pseudo-range model andpseudo-range rate is performed at the mobile GPS receiver.

Krasner (U.S. Pat. No. 6,064,336) has taught that the availability ofDoppler information can aid the mobile GPS receiver by reducing thefrequency uncertainty. U.S. Pat. No. 6,064,336 describes a system andmethod for delivering to a mobile receiver Almanac information fromwhich Doppler may be derived; or for delivering equivalent information,derived from the Almanac; or for delivering the Doppler measurementitself from a base station near to the mobile receiver. In anotheralternative embodiment of the current invention, the Ephemeris may beused to derive Doppler information. In the section that follows (SIGNALDETECTION) it will be appreciated that the use of this Dopplerinformation will aid in signal acquisition to the extent of reducing thePseudo-range rate uncertainty, i.e., the number of frequency bins tosearch, but the Doppler information will not reduce the Pseudo-rangeuncertainty (i.e. the code delays).

Signal Detection

There are several ways in which the availability of ephemeris data (orthe derived pseudo-range model) aid the signal acquisition andsensitivity of the mobile GPS receiver, described below.

The ephemeris or pseudo-range models can predict the elevation angle tothe satellite, allowing the receiver to focus on acquiring highelevation satellite signals, which are generally less subject toobstruction. Satellites that are calculated to be below the horizon(negative elevation angles) can be ignored. This satellite selection canalso be performed using an almanac of satellite orbital information, butproviding models, or ephemeris from which models can be created,eliminates the need for non-volatile storage for the almanac within themobile receiver. Thus, the ephemeris provides some advantage in thisrespect, however the main advantage of the invention is in theimprovement in signal acquisition and receiver sensitivity, describedbelow.

The “re-transmitted” or “re-broadcast” ephemeris information improvesthe operation of the mobile receiver in two ways.

First, the mobile receiver does not need to collect the ephemeris fromthe satellite. The ephemeris is broadcast from a satellite every 30seconds and requires 18 seconds to transmit. In order to receiveephemeris without the use of the present invention, a mobile receiverneeds clear, unobstructed satellite reception for the entire 18-secondinterval during which the ephemeris is being transmitted. Depending onthe environment and usage of the receiver, it may be minutes before thesituation allows the ephemeris to be collected and in many applications,for example, indoor use, the mobile receiver may never have anunobstructed view of a satellite. To eliminate the data collectiondelay, the present invention provides the ephemeris data directly to themobile receiver.

Second, the ephemeris is used, as described above, to form thepseudo-range models of the satellite signals being received at themobile receiver. These models can accelerate the acquisition process inseveral ways.

The models predict the pseudo-range and pseudo-range rate of thereceived signals. If the approximate user position is fairly accurate,these models will be very accurate in estimating the pseudo-range andpseudo-range rate. Using the models, the receiver can focus thecorrelation process around an expected signal.

FIG. 6 shows a graph 601 that illustrates the usual frequency and timinguncertainty for a mobile GPS receiver. On the y-axis 602, the variousrows show different pseudo range rates, and on the x-axis 604 thevarious columns show different pseudo ranges. Without an accurate model,such as available using the present invention, the possibilities forrange rates will vary considerably because a wide range of satellitemotions are possible, and the possibilities for ranges will also varyover many cycles of the PN codes. With an accurate model provided by theephemeris information, the uncertainties can be reduced to a smallrange, depicted by the black cell 606. Many receivers will be able tosearch this small range in a single pass that eliminates a timeconsuming sequential search and allows the use of longer integrationtimes for better sensitivity, as will now be described.

Better sensitivity is achieved as follows: The sensitivity of a GPSreceiver is a function of the amount of time that the receiver canintegrate the correlator outputs. The relationship between sensitivityand integration time is shown by the graph 608. With many bins tosearch, the integration time 610 equals the total available search timedivided by the number of search bins. With only a single bin to search,the integration time 612 equals the total available search time,increasing the sensitivity as shown 608.

It should be noted that in some receivers, the pseudo-ranges andpseudo-range rates that can be predicted from the pseudo-range modelswill not be accurate because of a lack of synchronization of the localclock. In this case, a search over a wide range of uncertainties willstill be initially required, but only for the strongest satellite(s). Ifthe local clock is known to be correct to within approximately onesecond of GPS time then any one satellite will be enough to synchronizethe local correlator offset. Thereafter, the expected pseudo-range andpseudo-range rates can be accurately computed for the remainingsatellites. If the local clock is not known to within approximately onesecond, then two satellites must be used to compute the two requiredclock parameters: the local clock and the correlator offset. The factthat two satellites are required is an often misunderstood point. In theGPS literature, it is often mentioned that one satellite is enough tosolve for an unknown clock offset without realizing that this is onlytrue for systems where the local clock is already approximatelysynchronized with GPS time. In traditional GPS receivers thatcontinuously track the GPS signals, the local clock is synchronized toGPS time to much better than one second accuracy. In some more modernimplementations (e.g., U.S. Pat. No. 6,064,336), the local clock issynchronized to a network time reference, which is synchronized to GPStime. However, the current invention is specifically intended to operatein implementations where the local clock is not synchronized to GPStime. The manner in which one solves for these clock parameters isdescribed in detail below.

Once the unknown clock parameters have been computed, the parameters canthen be used to adjust the pseudo-range models for the remaining, weakersatellites to reduce the range of uncertainty back to a narrow region;thus enhancing sensitivity precisely when high sensitivity is needed,i.e., for detecting the weaker satellite signals.

In other receivers, the local clock and clock rate may be quiteaccurate. For example, if the clock signals are derived from a wirelessmedia that is synchronized to GPS timing (e.g., a two-way pagingnetwork), then the clock parameters are typically accurate. In thiscase, there is no clock effect and a narrow search region can be usedfrom the onset.

To quantify the benefits of the invention, consider an example where theuser position is known to within the radius of reception of a 2-waypager tower (2-miles). In this case the pseudo-range (expressed inmilliseconds) can be pre-calculated to an accuracy of one-hundredth of amillisecond. Without the invention, a GPS receiver would search over afull millisecond of all possible code delays to lock onto the codetransmitted by the satellite. Using the invention the search window isreduced by up to one hundred times, making the GPS receiver faster, and,more importantly, allowing the use of longer integration times (asdescribed above), making the receiver capable of detecting weakersignals, such as occur indoors.

An additional advantage of having ephemeris, or the derived pseudo-rangemodel, at the mobile receiver is that the process of identifying thetrue correlation is more robust, since, apart from increasing theintegration time as described above, the chance that a “false peak”would be identified is greatly reduced by considering only correlationsthat occur within the expected range.

One embodiment of the use of ephemeris (or derived pseudo-range models)to enhance sensitivity is described further with respect to FIG. 7.

FIG. 7 is a flow diagram of a method 700 of signal search. The methodbegins at step 702 with an input of the pseudo-range model. As notedearlier this pseudo-range model is calculated from the ephemeris, eitherat the mobile receiver itself, or at the central processing site. Atstep 704, the model is applied at the current time in the mobile deviceand is used to estimate the expected current frequency and timing of GPSsatellite signals, as well as the expected uncertainties of thesequantities, to form a frequency and code delay search window for eachsatellite. This window is centered on the best estimates of frequencyand delay but allows for actual variations from the best estimates dueto errors in the modeling process including inaccuracies in the roughuser position, errors in the time and frequency transfer from thewireless carrier etc. In addition, the frequency uncertainty is dividedinto a number of frequency search bins to cover the frequency searchwindow. As shown in FIG. 6, the number of search bins is dramaticallyreduced by using the pseudo-range model.

In step 706, the detection and measurement process is set to program thecarrier correction to the first search frequency. At step 708, a codecorrelator is invoked to search for signal correlations within the delayrange of the delay window. Such a code correlator is standard in theart, but the present invention dramatically reduces the number ofpossible code delays over which the correlator must search therebyincreasing the integration time for each code delay, and thus thesensitivity of the receiver.

At step 710, the method 700 queries whether a signal is detected. If nosignal is detected, the carrier correction is set, at step 712, to thenext search frequency and the search continues until a signal is foundor the frequency search bins are exhausted.

If, at step 710, the method 700 affirmatively answers the query, thesignal is used at step 714 to further improve the estimate of clock timedelay and clock frequency offset. This information is utilized at step716 to re-compute the frequency and delay search windows for theremaining undetected satellites. In step 718, the process continuesuntil all satellites have been detected or the search windows have beenexhausted.

The method of FIG. 7 is illustrative of one of a variety of algorithmsthat can be used to guide the search process based on the GPS signalprocessing's ability to estimate time and frequency. Additionally, thealgorithms could be altered to include various retry mechanisms sincethe signals themselves may be fading or blocked.

Sensitivity Enhancement

To enhance the sensitivity of the receiver (as described with respect toFIG. 6), the invention uses the approximate position of the mobiledevice to compute expected pseudo-range, this reduces the pseudo-rangeuncertainty. However, before the inventive receiver can compute theexpected pseudo-range the following three items are required:

-   -   the approximate position of the mobile device (to within a few        miles of a true position)    -   the approximate time at the mobile device (to within        approximately one second of the true time)    -   the correlator clock offset at the mobile device (to within a        few microseconds of the true offset).

The more accurately each of the three terms is known, the more preciselythe invention can bound the pseudo-range uncertainty, and thus thegreater the sensitivity (see FIG. 6). In the preferred embodiment, theapproximate position of the mobile device is determined from the knownlocation of the radio tower last used by the device. The radius ofreception of radio towers for 2-way pagers and cell-phones is typically3 kilometers. Thus the approximate position of the mobile device isknown to within 3 kilometers, and the induced error on the pseudo-rangeestimate will be no more than 3 kilometers. With reference to FIG. 6,note that the full pseudo-range uncertainty for an unaided GPS receiveris equal to one code epoch, which is approximately 300 kilometers. Thus,even knowing an approximate position as roughly as 3 kilometers canreduce the pseudo-range uncertainty one hundred times.

The timing errors also induce errors on the expected pseudo-range. Tocompute expected pseudo-range, the receiver must calculate the satelliteposition in space. The satellite range from any location on earth variesat a rate between plus and minus 800 meters per second. Thus each secondof time error will induce a range error (and pseudo-range error) of upto 800 meters.

The mobile device correlator delay offset induces a direct error in thepseudo-range measurement, as is well known in the GPS literature. Eachmicrosecond of unknown correlator delay offset induces 300 meters oferror in the range measurement.

Thus, to keep the pseudo-range estimate within a range of a fewkilometers (as illustrated in FIG. 6), the receiver of the presentinvention requires estimates of position, time and correlator delayoffset in the ranges shown above.

In an implementation where the real time at the mobile device is notknown to better than a few seconds, and the correlator delay offset isnot known, one solves for both using two satellite measurements, asfollows.

The equation relating pseudo-range errors to the two clock errors is:y=c*dt _(c)−rangeRate*dt _(s)  (1)

where:

-   -   y is the “pseudo-range residual”, i.e., the difference between        the expected pseudo-range and the measured pseudo-range;    -   c is the speed of light;    -   dt_(c) is the correlator delay offset; and    -   dt_(s) is the offset of the real time estimate.

FIG. 8 depicts a flow diagram of a method 800 for improving the clockparameters, and then improving the receiver sensitivity. Method 800comprises:

Step 802. Using the best known clock parameters, compute expectedpseudo-ranges for all the satellites.

Step 804. Measure the pseudo-ranges for the two strongest satelliteswith the highest signal strength.

Step 806. Using these two measurements, solve equation (1) for the twounknowns: dt_(c) and dt_(s).

Step 808. Use dt_(c) and dt_(s) to improve the estimate of the expectedpseudo-ranges for the remaining (weaker) satellites.

Step 810. Use these improved expected pseudo-ranges to reduce thepseudo-range uncertainty, thus improving the sensitivity of thereceiver, as shown in FIG. 6.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

1. A method of distributing satellite-navigation data, comprising:processing satellite signals at each of a plurality of referencestations to receive a respective plurality of satellite-navigation-datastreams; forming packets in response to said plurality ofsatellite-navigation-data streams to generate a plurality of packetizedsatellite-navigation-data streams; sending each of said plurality ofpacketized-satellite-navigation-data streams to a processing system;removing, at said processing system, duplicate packets within saidplurality of packetized satellite-navigation-data streams to generate acombined packet stream; and sending said combined packet stream into acommunication network.
 2. The method of claim 1, further comprising:decoding satellite-navigation-data within said combined packet stream togenerate satellite data.
 3. The method of claim 2, wherein saidsatellite data comprises at least one of ephemeris data, almanac data,ionosphere data, universal time offset data, satellite health data, andraw data bits.
 4. The method of claim 1, wherein said plurality ofsatellite-navigation-data streams comprises global positioning system(GPS) satellite navigation messages, and wherein each of said packetsincludes a sub-frame of said GPS satellite navigation messages.
 5. Themethod of claim 4, wherein removing said duplicatesatellite-navigation-data streams comprises removing saidsatellite-navigation-data streams as a function of a satelliteidentifier.
 6. The method of claim 5, wherein removing said duplicatesatellite-navigation-data streams comprises removing saidsatellite-navigation-data streams as a function of said satelliteidentifier and a time-of-week value.
 7. The method of claim 1, whereinsaid processing system comprises a position-processing site, and themethod further comprises: receiving said combined packet stream fromsaid communication network at the position-processing site.
 8. Themethod of claim 7, further comprising: decoding satellite-navigationdata within said combined packet stream to generate satellite data; andstoring said satellite data at said position-processing site.
 9. Themethod of claim 7, further comprising: receiving, at saidposition-processing site, at least one additional packetizedsatellite-navigation-data stream; removing duplicate packets within saidcombined packet stream and said at least one additional packetizedsatellite-navigation-data stream to generate another combined packetstream; decoding satellite-navigation-data within said other combinedpacket stream to generate satellite data; and storing said satellitedata at said position-processing site.
 10. The method of claim 9,wherein said at least one additional packetizedsatellite-navigation-data stream is generated by at least one referencestation disposed proximate to said position-processing site.
 11. Asystem for distributing satellite-navigation data, comprising: aplurality of reference stations operable to process satellite signals toreceive a respective plurality of satellite-navigation-data streams, andto form packets in response to said plurality ofsatellite-navigation-data streams so as to generate a plurality ofpacketized-satellite-navigation-data streams; and a processing systemoperable to receive each of said plurality ofpacketized-satellite-navigation-data streams, to remove duplicatesatellite-navigation-data streams within said plurality of packetizedsatellite-navigation-data streams so as to generate a combined packetstream, and to send said combined packet stream into a communicationnetwork.
 12. The system of claim 11, wherein said processing systemincludes a processor operable to decode satellite-navigation data withinsaid combined packet stream to generate satellite data.
 13. The systemof claim 12, wherein said satellite data comprises at least one ofephemeris data, almanac data, ionosphere data, universal time offsetdata, satellite health data, and raw data bits.
 14. The system of claim11, wherein said plurality of satellite-navigation-data streamscomprises global positioning system (GPS) satellite navigation messages,and wherein each of said packets includes a sub-frame of said GPSsatellite navigation messages.
 15. The system of claim 14, wherein theprocessor is operable to remove said duplicativesatellite-navigation-data streams as a function of a satelliteidentifier.
 16. The system of claim 15, wherein the processor isoperable to remove said duplicative satellite-navigation-data streams asa function of any of said satellite identifier and a time-of-week value.17. The system of claim 11, further comprising: a position-processingsite operable to receive said combined packet stream.
 18. The system ofclaim 17, wherein said position-processing site comprises: a processorfor decoding satellite-navigation data within said combined packetstream to generate satellite data, and a memory for storing saidsatellite data.
 19. The system of claim 17, further comprising: anadditional reference station disposed proximate to saidposition-processing site and operable to provide at least one additionalpacketized-satellite-navigation-data stream; wherein saidposition-processing site comprises: a processor operable to removeduplicate satellite-navigation-data streams within said combined packetstream and said at least one additional packetizedsatellite-navigation-data stream to generate another combined packetstream, and to decode satellite-navigation-data within said othercombined packet stream to generate satellite data; and a memory forstoring said satellite data.
 20. An apparatus for distributingsatellite-navigation data, comprising: means for processing satellitesignals at each of a plurality of reference stations to receive arespective plurality of satellite-navigation-data streams; means forforming packets in response to said plurality ofsatellite-navigation-data streams to generate a plurality of packetizedsatellite-navigation-data streams; means for sending each of saidplurality of packetized satellite-navigation-data streams to aprocessing system; means for removing, at said processing system,duplicate packets within said plurality of packetizedsatellite-navigation-data streams to generate a combined packet stream;and means for sending said combined packet stream into a communicationnetwork.